Externally threaded resin terminal bushing having a floating ground shield



1966 c. F. SONNENBERG ET AL 3,230,301

EXTERNALLY THREADED RESIN TERMINAL BUSHING HAVING A FLOATING GROUND SHIELD Filed July 12, 1963 F ig.l.

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SHIELD WITNESSES INVENTORS Charles E Sonnenberg $26M and Donald J. Murfohus W /P- M ATTORNEY United States Patent EXTERNALLY THREADED RESIN TERMINAL BUSHING HAVING A FLOATING GROUND SHIELD Charles F. Sonnenberg, Penn Hills Township, Allegheny County, and Donald J. Martahus, Export, Pan, assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed July 12, 1963, Ser. No. 294,709 5 Claims. (Cl. 174-143) This invention relates generally to terminal bushings and, more particularly, to terminal bushings which are molded from a cast resin, and to a method for fabricating the same.

A general object of the present invention is to provide an improved cast-resin type of terminal bushing of improved operating characteristics and of economical manufacture.

A more specific object of the present invention is the provision of an improved resinous-type terminal bushing, in which improved means are provided for securely attaching a ground flange thereto, which will be capable of resisting vibration and shock during service conditions.

Another object of the present invention is the provision of an improved cast-resin type of terminal bushing in which improved ground shielding means is provided to relieve the voltage stress at the inner periphery of the associated ground flange.

Still a further object of the present invention is the provision of an improved terminal bushing of the cast-resin type, in which an embedded ground shield is provided and disposed in such immediate proximity to the surrounding grounding flange as to be substantially at ground potential.

Still a further object of the present invention is the provision of an improved resinous-type terminal bushing in which attaching threads are molded on the external surface of the resinous bushing body so as to cooperate with an associated metallic grounding flange having a cooperating thread structure.

Yet a further object of the present invention is the provision of an improved resinous-type terminal bushing in which the generation of radio-interfering voltage is avoided by an improved near-ground floating shield embedded in the cast resin.

In the casting of resinous-type terminal bushings, say of the epoxy-resin type, one of the most frustrating problems to date has been the obtaining of a homogeneous structure, which will remain bonded to both the central conducting terminal stud and also to the outer ground flange sleeve without corona-producing microscopic voids. Such intimate bonding must remain void-free after an extended series of temperature cycles from -40 C. to +100 C. to insure no internal corona formation after an extended period of service.

An early attempt to eliminate these corona-producing voids saw the use of aluminum conductors and aluminum ground sleeves, since most filled epoxy resins and aluminum have almost identical thermal coeflicients of expansion. Difficulty was, however, encountered in that the shrinkage during cure of the resin created voids under the encircling ground flange sleeve. As set forth in US. Patent 3,001,005, issued September 19, 1961, to Charles F. Sonnenberg, a method of slotting the aluminum ground sleeves and the utilization of flexible resin liners were described. The flexibility of the slotted aluminum sleeve permitted complete adhesion to the shrinking resin, and thus was a rather effective means to eliminate the coronacausing voids. However, a disadvantage of the slotting construction was the concomitant shop operations which had to be performed, which, of course, were rather ex- 3,230,301 Patented Jan. 18, 1966 pensive. As a result, the use of a slotting aluminum ground flange confined the adoption of the cast resin bushings to the higher voltage ranges, and prevented their use in the relatively inexpensive low-voltage class of terminal bushings.

In the aforesaid patent to Sonnenberg there was also disclosed and claimed the concept of burying a sleeve, or perforated cylinder into the cast resin terminal bushing body. A connection from such a buried sleeve was made to the terminal bushing mounting bolts through an interconnected horizontal conducting screen. Such an idea provided the advantages of a buried grounding sleeve with a cast resin mounting flange, the horizontal screen also mechanically serving the function of reinforcing the cast grounding flange. Perforations in the shield, or the use of screening, allowed the resin to cohere through the metallic shield and effectively eliminated voids in the high-gradient area on the shield outside surface. Here again, however, the effort required to insert the perforated shield and make a positive electrical connection during the pouring and molding operation was quite costly. In addition, the mold and other related tooling and fixtures created a rather expensive manufacturing operation.

It is, accordingly, a further object of the present invention to surmount the aforesaid problems and to provide a highly efficient and economical resinous-type terminal bushing suitable for high-quantity manufacture and increasing the corona-inception voltages to avoid radio interference.

Still a further object of the invention is to provide an improved method of manufacturing cast-resin type terminal bushings.

Further objects and advantages will readily become apparent upon reading the following specification, taken in conjunction with the drawing, in which:

FIGURE 1 is a vertical sectional view taken through a cast-resin type of terminal bushing embodying features of the present invention; and,

FIG. 2 is a side elevational view, partially in vertical section, of a modified type of construction suitable for outdoor service conditions.

Referring to the drawing, and more particularly to FIG. 1 thereof, the reference numeral 1 generally designates a cast-resin type of terminal bushing. As shown, the terminal bushing 1 comprises an axially-extending conductor stud 2 surrounded by a cast resin body 3. The terminal bushing 1 is preferably cast about the central metallic conductor stud 2 by molding means, such as those set forth in the aforesaid Patent 3,001,005 to Sonnenberg. The use of aluminum for the central conductor stud 2 is desirable because of the similarity of thermal coefficients of expansion, but is somewhat optional, since, being internal to the resinous body 3, there exists enough compressive curing stress, which can be designed into the resin system, to overcome any later contractual tendencies during low temperatures.

A substantially cylindrical exterior portion may be designed into the mold over say, for example, about a 4-inch central section, where a mounting flange is desired. Here, there is preferably utilized molding such as to produce a coarse helical thread 6 similar to that molded into metal bottle covers. At a diameter slightly less than the root diameter of these threads 6 will be placed an electrically floating screen, or perforated shield 9, disposed slightly inwardly to provide the desired ground layer stress. Although this shield is electrically floating, nevertheless, the close proximity to the external mounting flange 12 insures that by capacitive coupling through the very thin cylinder of resin 14 that the induced voltage of the shield 9 cannot be significantly different from that of the immediately surrounding grounding mounting flange 12. If, by accident, the two should contact, absolute grounding is assured.

After the cast assembly has been cured to achieve the minimum outside diameter resulting from shrinkage, the metallic flange 12 is provided with matching bottle caplike threads and is coated on the inside with an epoxy adhesive 13 and threaded onto the resin bushing body 3. The adhesive between the cast resin and flange sleeve could be any one of the flexible epoxies marketed by the Furane Plastics Company under the trade name Epobond. Such adhesives would be most suitable, but, of course, other adhesives, Well-known to those skilled in the art could be utilized.

The thermoset resin utilized for the body 3 should be one that has good physical strength, has a relatively low shrinkage during curing and has good adherence to metal and ceramics, such as porcelain and glass. The resinous polymeric expoxides, such as a thermosetting resin comprising a glycidyl polyether of a polyhydric phenol having a 1, 2 epoxy equivalence of between 1 and 2, have been found to meet the foregoing requirements and have been successfully utilized in making bushings of the present type. Such epoxide resins are described in US. Patents 2,728,744 and 2,739,134.

Since most terminal bushings require a ground sleeve length of 12 to 16 inches, it is very uneconomical to re quire the flange sleeve to be this long as it rules out the use of one-piece castings unless the diameter is inches, or greater. By use of the buried-sleeve technique, the actual flange 12 need only be 4 to 5 inches in axial length, and can thus be die cast, etc. Such die-cast flanges may be readily purchased complete for $0.50, whereas longer fabricated flanges may cost $3.50 in accordance with quotations from the Latrobe Die Casting Company.

Tests on buried near-ground sleeves 9 within the laboratory have shown corona-inception voltages above the rated line-to-ground voltage of the kv. cast resin bushings, as resulting corona is very low at voltages above the inception voltage (15-20 picocoulombs at kv.). As a result, it is possible to develop a low shrinkage, nonvacuum requiring resin in an economical type of terminal bushing 1.

By use of the aforesaid constructions, it is possible to manufacture a cast resin bushing, which can be produced in quantity at a cost less than that of the present shellactreated paper bushings.

For outdoor service, a porcelain weather casing 21 and cap 23 may be attached to the upper portion of the cast resin bushing body, as illustrated in FIG. 2 of the draw ings. The space between the porcelain weather casing and the upper portion of the cast resin bushing body may be filled with either an adhesive or a self-retaining flexible and resilient member 22, which may assume the form of a double flanged frustrum of a right circular cone. The inside diameter of the cone would be slightly less than the outside diameter of the cast resin body portion 3 of the bushing 1, and the outside diameter would be slightly less than the inside diameter of the porcelain casing 21, as set forth in the aforesaid Patent 3,001,005. The porcelain weather casing would assist in adapting the terminal bushing of the present application to outdoor service conditions. For indoor service, on the other hand, such a porcelain weather casing 21 could be omitted.

From the foregoing description of the invention it will be apparent that there is provided an improved cast-resin type of terminal bushing having improved means for fixedly securing the ground flange assembly thereto to withstand shock and vibration in service. As well-known by those skilled in the art, for circuit-breaker application, the arc-extinguishing unit is fixedly secured to the interior end of the terminal stud, and during opening and closing operations, considerable cantilever stresses are im posed upon the terminal bushing structure. It is, therefore, essential for the metallic ground flange 12 to be securely attached to the bushing body 3 to withstand such severe usage.

Although there has been illustrated and described specific structures, it is to be clearly understood that the same were merely for the purpose of illustration, and that changes and modifications may readily be made therein by those skilled in the art without departing from the spirit and scope of the invention.

We claim as our invention:

1. A terminal bushing structure including a bushing body composed at least partially of a thermosetting resin, said bushing body having external threads formed adjacent the central portion thereof, a cooperating metallic ground flange having matching threads, the bushingbody being threadedly secured to the metallic ground flange by the threaded interconnection, an adhesive on the threaded portion of the outside of the bushing body interposed between the ground flange and the bushing body and preventing the threads of the metallic ground flange from setting up stresses in the bushing body which result in irregular gradients in the electrostatic field therein and corona producing microscopic nonuinformities therein, and an embedded cylindrical ground shield composed of conductive screening material in said bushing body adjacent the threads thereon and having a diameter less than the root diameter of said last-named threads, the length of the ground shield being at least as great as the length of the threaded portion of the bushing body.

2. A terminal bushing structure according to claim 1 wherein the ground shield is additionally characterized as being electrically floating, the ground shield being disposed near the last-named threads whereby capacitive coupling from the ground shield to the ground flange results in an induced voltage on the ground shield which places the ground shield at substantially ground potential.

3. A terminal bushing structure including a bushing body composed at least partially of a thermosetting resin and a conductor stud extending therethrough, said bushing body having external threads formed adjacent the central portion thereof, a cooperating metallic ground flange having matching threads, the bushing body being threadedly secured to the metallic ground flange by the threaded interconnection, an adhesive on the threaded portion of the outside of the bushing body interposed between the ground flange and the bushing body and preventing the threads of the metallic ground flange from setting up stresses in the bushing body Which result in irregular gradients in the electrostatic field therein and corona producing microscopic nonuniformities therein, and an embedded electrically floating cylindrical ground shield composed of conductive screening material in said bushing body, the diameter of the ground shield being less than the root diameter of the threads on the bushing body, the ground shield being disposed near the last-named threads whereby capacitive coupling between the ground shield and the ground flange results in an induced voltage on the ground shield which places the ground shield at substantially ground potential.

4. A terminal bushing structure including a bushing body composed at least partially of a thermosetting resin and a conductor stud extending therethrough, said bushing body having external threads formed adjacent the central portion thereof, a cooperating metallic ground flange having matching threads, the bushing body being threadedly secured to the metallic ground flange by the threaded interconnection, an adhesive on the threaded portion of the outside of the bushing body interposed between the ground flange and the bushing body and preventing the threads of the metallic ground flange from setting up stresses in the bushing body which result in irregular gradients in the electrostatic field therein and corona producing microscopic nonuniformities therein, and an embedded electrically floating cylindrical ground shield composed of conductive screening material in said bushing body adjacent the threads thereon, the electrically floating ground shield extending beyond the ends of the threaded portion and resulting in a distribution of the electrostatic field between the conductor stud and the ground flange which reduces the electric stress in the area near the ground flange.

5. A terminal bushing structure including a bushing body composed at least partially of a thermosetting resin and a conductor stud extending therethrough, said bushing body having external threads formed adjacent the central portion thereof, a cooperating metallic ground flange having matching threads, the bushing body being threadedly secured to the metallic ground flange by the threaded interconnection, an adhesive on the threaded portion of the outside of the bushing body interposed between the ground flange and the bushing body and preventing the threads of the metallic ground flange from setting up stresses in the bushing body which result in irregular gradients in the electrostatic field therein and corona producing microscopic nonuniformities therein, and an embedded electrically floating cylindrical ground shield composed of conductive screening material in said bushing body adjacent the threads thereon, the diameter of the ground shield being less than the root diameter of the last-named threads, the ground shield being disposed References Cited by the Examiner UNITED STATES PATENTS 1,497,952 6/1924 Smith. 1,871,356 8/1932 Bakken 174-152 X 2,630,469 3/1953 Miller 174153 X 3,001,005 9/1961 Sonnenberg 174-142 FOREIGN PATENTS 872,002 7/1961 Great Britain. 7 876,851 9/ 1961 Great Britain.

JOHN F. BURNS, Primary Examiner.

L. E. ASKIN, Examiner, 

1. A TERMINAL BUSHING STRUCTURE INCLUDING A BUSHING BODY COMPOSED AT LEAST PARTIALLY OF A THERMOSETTING RESIN, SAID BUSHING BODY HAVING EXTERNAL THREADS FORMED ADJACENT THE CENTRAL PORTION THEREOF, A COOPERATING METALLIC GROUND FLANGE HAVING MATCHING THREADS, THE BUSHING BODY BEING THREADEDLY SECURED TO THE METALLIC GROUND FLANGE BY THE THREADED INTERCONNECTION, AN ADHESIVE ON THE THREADED PORTION OF THE OUTSIDE OF THE BUSHING BODY INTERPOSED BETWEEN THE GROUND FLANGE AND THE BUSHING BODY AND PREVENTING THE THREADS OF THE METALLIC GROUND FLANGE FROM SETTING UP STRESSES IN THE BUSHING BODY WHICH RESULT IN IRREGULAR GRADIENTS IN THE ELECTROSTATIC FIELD THEREIN AND CORONA PRODUCING MICROSCOPIC NONUNIFORMITIES THEREIN, AND AN EMBEDDED GROUND SHIELD COMPOSED OF CONDUCTIVE SCREENING MATERIAL IN SAID BUSHING BODY ADJACENT THE THREADS THEREON AND HAVING A DIAMETER LESS THAN THE ROOT DIAMETER OF SAID LAST-NAMED THREADS, THE LENGTH OF THE GROUND SHIELD BEING AT LEAST AS GREAT AS THE LENGTH OF THE THREADED PORTION OF THE BUSHING BODY. 